1. Field of the Invention
The present invention relates to key input devices for use in portable telephones and the like, and more particularly to a key input device comprising a plurality of pairs of manual keys including a seesaw key adapted to selectively close one of the pairs of key contacts.
2. Description of the Related Art
With reference to FIG. 8, portable telephones usually have a casing formed by a front case and a rear case. A display and a plurality of manual keys are arranged on the surface of the front case. These manual keys include a seesaw key for use in selecting one of a plurality of functions. The illustrated seesaw key is adapted to selectively close one of four pairs of key contacts for the selection of the corresponding function.
The casing has a circuit board disposed in its interior as shown in FIG. 9. Arranged on the circuit board are a key sheet assembly to be depressed by the manual keys, the display, a high-frequency circuit module, a vibrator comprising a motor and an eccentric weight attached to the output shaft of the motor for notifying the user of incoming calls, etc.
With reference to FIG. 10, the key sheet assembly has a dome sheet of PET disposed over a base plate, with a spacer sheet interposed therebetween, and having a plurality of upwardly bulging dome portions. The manual keys including the seesaw key are arranged above the dome sheet for depressing the respective dome portions. The spacer sheet is formed with a plurality of circular holes corresponding to the respective dome portions and slotlike air vents interconnecting these circular holes, as shown in FIG. 11.
With the key sheet assembly, the dome portion is elastically restorably inverted by being depressed by the corresponding manual key to bring a contact terminal provided on the inner surface of the dome portion into contact with a contact electrode provided on the surface of the base plate to close a pair of contacts.
To reliably close the key contacts of the key sheet assembly, the contact terminal on the inner surface of the dome portion of the dome sheet must be brought into contact with the contact electrode on the surface of the base plate over as large an area as possible. It is therefore necessary to depress the dome portion vertically downward.
However, in the case of the dome portions to be depressed by the seesaw key, a pusher projecting downward from an end portion of the seesaw key depresses the corresponding dome portion in an oblique direction when the seesaw key is moved. Stated more specifically with reference to FIG. 10, the pusher of the seesaw key exerts a depressing force F at an angle of xcex8 with a center axis V extending through the dome portion perpendicular thereto, consequently deforming the dome portion to a distorted form and bringing the contact terminal on the inner surface of the dome portion into point contact with the contact electrode on the base plate, hence the problem of failing to close the key contacts reliably.
Accordingly, an object of the present invention is to provide a key input device which is adapted to close key contacts reliably even with a seesaw key.
The present invention provides a key input device which includes a dome sheet disposed over a base plate and having a plurality of upwardly bulging dome portions, and a plurality of manual keys arranged above the dome sheet for depressing the respective dome portions, the manual keys including a seesaw key adapted to selectively depress one of dome portions included among the dome portions, each of the dome portions being elastically restorably invertable by being depressed by the corresponding manual key to bring a contact terminal formed on an inner surface of the dome portion into contact with a contact electrode formed on a surface of the base plate to close contacts.
The dome sheet has a flat sheet portion formed around the dome portions and comprising an inner region extending inwardly of the dome portions to be selectively depressed by the seesaw key, and an outer region extending outwardly of the dome portion. The seesaw key is formed with a pivotal pin projecting toward the inner region of the dome sheet. A spacer sheet is interposed between the base plate and the dome sheet and is formed with a plurality of holes corresponding to the respective dome portions. The interposed spacer sheet provides a difference in level from the surface of the base plate so that the inner region is positioned at a lower level than the outer region on the flat sheet portion of the dome sheet. The seesaw key pivotally moves about the contact portion of the pivotal pin and the inner region of the dome sheet to depress any one of dome portions.
Because the inner region of the flat sheet portion of the dome sheet is at a lower level than the outer region thereof in the key input device of the present invention, each of the dome portions to be selectively depressed by the seesaw key is included so as to be positioned at a lower level toward the inner region and at a higher level toward the outer region. Thus, each dome portion has its center axis included with respect to a perpendicular to the surface of the base plate. It is not always necessary to provide the difference in level as described above between the inner region and the outer region on the flat sheet portion of the dome sheet, but the difference should be provided when the seesaw key is operated to depress any one of the dome portions.
When one of these dome portions is pushed by depressing the seesaw key, the seesaw key pivotally moves about the contact portion of the pivotal pin and the inner region, and the center axis C extending through the dome portion perpendicular thereto is accordingly positioned in the same or approximately the same direction as the direction in which the depressing force F of the seesaw key acts. As a result, the dome portion is deformed symmetrically about the center axis, bringing the contact terminal on the inner surface of the dome portion into contact with the corresponding contact electrode on the base plate over a sufficient area. Because the dome portion deforms symmetrically about the center axis, this portion enables the user to feel the same sharp click as when with the other manual keys are depressed.
According to a specific embodiment of the present invention, the spacer sheet has an opening formed inwardly of holes corresponding to the respective dome portions to be selectively depressed by the seesaw key and including the holes, and the dome sheet has its flat sheet portion joined at the inner region to the surface of the base plate through the opening. In this embodiment, a level difference in match with the thickness of the spacer sheet is provided between the inner region of the flat sheet portion of the dome sheet and the outer region thereof.
With the key input device of the present invention, the seesaw key, like the other keys, brings the contact terminal on the inner surface of the dome portion into contact with the corresponding contact electrode on the base plate over a sufficient area as described above, whereby the key contacts can be closed reliably.